[LEAPSECS] Schedule for success

Steve Allen sla at ucolick.org
Wed Dec 31 03:51:35 EST 2008

On Wed 2008-12-31T08:10:17 +0000, Poul-Henning Kamp hath writ:

> >But GMT has a

> >clear definition tied to - Hush my mouth! - the position of the mean

> >sun in the sky.


> No it does not, GMT wandered several seconds when the 0� longitude

> were moved away from he observatory.

GMT did not shift by seconds. As with the rest of the discussion here
we are talking about two different and necessary-to-distinguish

The geodetic longitude of the Greenwich transits is 5 arc seconds west
of the "International Meridian" defined by the global,
satellite-geocentered, VLBI-oriented reference frames. That's less
than one second of time -- but it's mostly irrelevant to GMT.

Of those 5 arcseconds difference, the major component is due to the
E/W component of the deflection of vertical at Greenwich. That is to
say, at Greenwich "up" is 5 arc seconds different than "geocentrically

Strict GMT is defined by the astronomical longitude of the

Yes, using the international reference frames there was a shift. When
BIH chose the statistical weights of the observations from all sites,
and as they averaged the differences in the "conventional longitudes"
of all the observatories which had been in use from the pre-satellite
era the terrestrial reference frame did shift.

This shift on the order of centiseconds of time.

And yes, it will drift over time at plate tectonic rates.
But that shift will not accelerate quadratically in the way that
atomic time and universal time will diverge.

Steve Allen <sla at ucolick.org> WGS-84 (GPS)
UCO/Lick Observatory Natural Sciences II, Room 165 Lat +36.99855
University of California Voice: +1 831 459 3046 Lng -122.06015
Santa Cruz, CA 95064 http://www.ucolick.org/~sla/ Hgt +250 m

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